1. Field of the Invention
The present invention relates to a film bulk acoustic wave resonator configured to trap the energy of an acoustic wave in a resonance portion.
2. Description of Related Art
The components incorporated in electronic devices such as portable devices are demanded to be smaller and lighter. For example, filters used in portable devices are demanded to be smaller and to be capable of precise adjustment of frequency characteristics. As a filter that satisfies these demands, a filter having a film bulk acoustic wave resonator is known.
A conventional film bulk acoustic wave resonator will be described below with reference to FIGS. 13, 14, 15A and 15B. FIG. 13 is a cross sectional view showing the basic structure of a conventional film bulk acoustic wave resonator 50. The film bulk acoustic wave resonator 50 has a resonance portion in which a piezoelectric body 51 is sandwiched between a lower electrode portion 52 and an upper electrode portion 53. This resonance portion is placed on a semiconductor substrate 55 in which a cavity 54 is formed. The cavity 54 can be formed by, for example, partially etching from the back surface of the semiconductor substrate 55 using a micromachining method.
As shown in FIG. 14, by applying an electric field in the thickness direction to the piezoelectric body 51 with the lower electrode portion 52 and the upper electrode portion 53, this film bulk acoustic wave resonator 50 produces vibrations in the thickness direction. The operation of this film bulk acoustic wave resonator 50 will be described using a thickness longitudinal vibration of an infinite plate. In the film bulk acoustic wave resonator 50, when an electric field is applied between the lower electrode portion 52 and the upper electrode portion 53, the electric energy is converted to mechanical energy by the piezoelectric body 51. The induced mechanical vibration is an extensional vibration in the thickness direction, and elongates and contracts in the direction of the electric field. Generally, the film bulk acoustic wave resonator 50 utilizes a resonant vibration in the thickness direction of the piezoelectric body 51, and is operated with a resonance frequency such that a half wavelength corresponds to a thickness of the piezoelectric body 51. The cavity 54 shown in FIG. 13 is utilized to ensure the thickness longitudinal vibration of this piezoelectric body 51.
An equivalent circuit of this film bulk acoustic wave resonator 50 has both series resonance and parallel resonance as shown in FIG. 15A. In other words, the equivalent circuit of this film bulk acoustic wave resonator 50 is composed of a series resonance portion including a capacitor C1, an inductor L1 and a resistance R1, and a capacitor C0 connected in series to the series resonance portion. With this circuit configuration, the admittance frequency characteristics of the equivalent circuit as shown in FIG. 15B are obtained: the admittance becomes the greatest at the resonance frequency fr, and becomes the smallest at the antiresonance frequency fa. The resonance frequency fr and the antiresonance frequency fa satisfy the following relationship.fr=1/{2π√(L1×C1)}fa=fr√(1+C1/C0)
When the film bulk acoustic wave resonator 50 having these admittance frequency characteristics is used as a filter, a small low-loss filter can be realized, because the resonant vibration of the piezoelectric body 51 is obtained. As shown in FIG. 16A, by connecting two film bulk acoustic wave resonators 50 in series and parallel, a band-pass filter having characteristics as shown in FIG. 16B can be formed easily.
Actually, the film bulk acoustic wave resonator is necessarily fixed to a substrate, and the size of electrodes in the radial direction is finite. Therefore not all the thickness longitudinal vibration generated by the vibration portion is excited as the main resonant vibration, but a part of the vibration leaks into the substrate or the piezoelectric body disposed outside the resonator. Because of this vibration leakage (unwanted vibration) to the substrate or in the radial direction, a part of the energy intended to be used to excite vibrations inside the piezoelectric body should be treated as a loss. To address this, inventions to reduce the energy loss are disclosed in WO 99/37023, JP 2003-505906 A and the like.
The conventional method of reducing an energy loss disclosed in JP 2003-505906 A will be described with reference to FIGS. 17A to 17C. This method employs an energy trapping structure, which is one of usual methods of reducing an energy loss. FIG. 17A is a plan view of a film bulk acoustic wave resonator. FIG. 17B is a side view showing a cross section viewed from the front of FIG. 17A. FIG. 17C is a cross sectional view taken from a side of FIG. 17A. Note that FIG. 17A does not show a substrate 60, which is shown in other diagrams.
A resonator structure is formed on the substrate 60 via an etch pit 61 and a film layer 62. The etch pit 61 is provided to insulate the resonator structure from the substrate 60. The resonator structure includes two conductive layers 63 and 64, and a piezoelectric layer 65 interposed therebetween. The conductive layers 63 and 64 as well as the piezoelectric layer 65 extend in a first region capable of piezoelectric excitation, and excitation can be performed in a specific piezoelectric excitation mode.
This resonator structure includes a frame zone 66 surrounding the center region within the first region. The frame zone 66 is formed by increasing part of the thickness of the upper conductive layer 64. The cutoff frequency of the layer structure of the frame zone 66 in a piezoelectric excitation mode differs from the piezoelectric excitation mode of the layer structure of the center region. The width of the frame zone 66 and the acoustic characteristics of the layer structure of the frame zone 66 are configured such that displacement of the maximum resonance mode excited by the piezoelectric body is almost uniform in the center region of the resonator. Thereby, excellent electric characteristics can be realized.
Another example of an energy trapping structure described in WO 99/37023 will be explained with reference to FIGS. 18A to 18D. FIG. 18A is a top view of a resonator. FIG. 18B is a cross sectional view of the resonator. FIG. 18C is an enlarged cross sectional view of an acoustic resonance portion. FIG. 18D shows dispersion curves of the resonator.
A film bulk piezoelectric element 70 (see FIG. 18B) includes a lower electrode 72 formed on one surface of a substrate 71, a piezoelectric thin film 73 formed on the lower electrode 72, and a first upper electrode 74 formed on the piezoelectric thin film 73. Further, a second upper electrode 75 having a mass load larger than that of the first upper electrode 74 is formed outside the first upper electrode 74 on the piezoelectric thin film 73. Reference numeral 76 denotes a cavity, reference numeral 77 denotes a leading wiring, and reference numeral 78 denotes a pad.
As shown in FIG. 18D, the piezoelectric thin film 73 exhibits a high-cut-type dispersion curve. In FIG. 18D, y1 represents the dispersion characteristic of a non-electrode portion piezoelectric body 79c (see FIG. 18C), y2 represents the dispersion characteristic of a first upper electrode portion piezoelectric body 79a that corresponds to the thin first upper electrode 74, and y3 represents the dispersion characteristic of a second upper electrode portion piezoelectric body 79b that corresponds to the thick second upper electrode 75. The cutoff frequency of the second upper electrode portion piezoelectric body 79b having a larger mass load can be lower than the cutoff frequency of the first upper electrode portion piezoelectric body 79a, and thus the energy of acoustic waves can be trapped in the region on the first upper electrode portion 79a side. Accordingly, vibration leakage can be reduced, and excellent performance can be realized.
According to the above-described inventions disclosed in JP No. 2003-505906A and International Publication WO 99/37023, the mass load electrode or the frame zone is disposed within the outline of the surface of the cavity in contact with the resonator, whereby the vibration is trapped in the center region before the vibration reaches the substrate to suppress the vibration leakage (unwanted vibration) from the vibration portion to the substrate. Therefore, excellent resonator characteristics and excellent filter characteristics can be expected.
However, in the above configuration disclosed in JP No. 2003-505906A or International Publication WO 99/37023, as shown in FIG. 19A, the frame zone 66/mass load electrode 75 is electrically connected to the center region. This causes a problem that, even in the frame zone 66/mass load electrode 75, as shown in FIG. 19B, resonance occurs in different resonance modes. In FIG. 19B, A represents the resonance mode of the center region, and B represents the resonance mode of the mass load region (frame zone 66/mass load electrode 75). This causes the electric energy to be dispersed in the center region (resonance mode A) and the mass load region (resonance mode B). As a result, the characteristics of the main resonance mode A are degraded.
In other words, in the case of forming a band-pass filter using a film bulk acoustic wave resonator, as in the above conventional examples, the problem also occurs that attenuation characteristics are degraded outside the pass band as shown in FIG. 19C, because each resonator has a different resonance mode.
WO 99/37023 further describes a film bulk acoustic wave resonator having a structure as shown in FIGS. 20A to 20B. This film bulk acoustic wave resonator basically has the same structure as shown in FIGS. 18A to 18C. The difference is that a dielectric 80 is provided outside a vibration portion made up of a pair of first upper electrodes 74. The dielectric 80 functions similarly to the second upper electrode 75 configured as shown in FIGS. 18A to 18C and acts as a mass load. In addition, the dielectric 80 is electrically insulated from the first upper electrode 74.
However, WO 99/37023 does not clearly disclose the requirements for the dielectric 80 to effectively allow the dielectric 80 to act as a mass load that reduces the vibration leakage by effectively trapping the energy of acoustic waves.